Global warming has become a serious problem because of an increase in CO2 concentration, which is due to the use of large amounts of fossil fuels, typified by petroleum, so far, and further, the depletion of fossil fuels has been feared. Therefore, how to meet the demand for a large amount of energy in the future has become a very important problem on a global scale. In such circumstances, using light energy, which is infinite and is clean against nuclear power generation, for power generation has been positively studied. As solar cells that convert solar energy into electric energy, inorganic solar cells using inorganic materials, such as single-crystal silicon, polycrystalline silicon, and amorphous silicon, and organic solar cells using organic dyes and conductive polymer materials have been proposed.
In such circumstances, a dye-sensitized solar cell (Gratzel type solar cell) (for example, see Patent Document 1 and Non Patent Document 1) proposed by Dr. Gratzel et al., Switzerland in 1991, has been expected as a next-generation solar cell because, in addition to a simple production process, the same level of conversion efficiency as amorphous silicon is obtained. The Gratzel type solar cell comprises a semiconductor electrode in which a semiconductor layer on which a dye is adsorbed is formed on a conductive basal material, a counter electrode composed of a conductive basal material opposed to the semiconductor electrode, and an electrolyte layer held between the two electrodes.
In the Gratzel type solar cell, the adsorbed dye absorbs light and is in an excited state, and electrons are injected into the semiconductor layer from the excited dye. The dye that is in an oxidized state by the emission of electrons is reduced by the transfer of electrons to the dye due to the oxidation reaction of the redox agent in the electrolyte layer, and returns to the original dye. The redox agent that has donated electrons to the dye is reduced again on the counter electrode side. The Gratzel type solar cell functions as a solar cell that converts light energy into electric energy by this series of reactions.
In the Gratzel type solar cell, by using for the semiconductor layer porous titanium oxide obtained by sintering fine particles, the surface area where the dye is adsorbed, that is, the effective reaction surface area, increases as much as about 1000 times. Compared with a case where a titanium oxide film fabricated by a vapor phase growth method is used, a great feature of the case where porous titanium oxide is used is that larger photocurrent is obtained.
In the Gratzel type solar cells, metal complexes, such as ruthenium complexes, are used as the sensitizing dye, and specifically, for example, a cis-bis(isothiocyanato)-bis-(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II) ditetrabutylammonium complex, bipyridine complexes of ruthenium, such as cis-bis(isothiocyanato)-bis-(2,2′-bipyridyl-4,4′-dicarboxylic acid)ruthenium(II), and a tris(isothiocyanato)(2,2′:6′,2″-terpyridyl-4,4′,4″-tricarboxylic acid)ruthenium(II) tritetrabutylammonium complex, which is one of terpyridine complexes of ruthenium, are used.